The recent introduction of a statutory distinction between natural flavourings and those that have an identical chemical structure but are synthesised (U.S. Code of Federal Regulations 21: 101.22.a.3, EC Directive 88/388 and D.L. 25 Jan. 1992 No. 107) has made it worthwhile to produce substantial quantities of non-accessible flavouring agents by extracting them from natural sources by biodegrading natural products (D. W. Armstrong in Flavor Chemistry, Trends and Developments, Ed. R. Teranishi, R. G. Buttery, F. Shahidi; ACS Symposium Series 383, American Chemical Society, Washington D.C., 1989, p.105-120). These products are preferred by the consumer and add value to the foods to which they are added; it is therefore desirable for them to be produced by these methods.
Some of the most important of these flavouring products are gamma and delta lactones from C.sub.6 to C.sub.12, which can be either saturated or mono- or polyunsaturated, with the unsaturation of the ring in various positions in the chain. These are the key constituents of many fruit flavourings and milk products. They are generated in fruit in extremely small quantities, generally at the time of ripening. It is uneconomical to extract lactones for use as flavourings from natural sources because of the low concentration in which they are present and because the concentration depends on the degree of ripening and many other factors that are difficult to control.
Hence, in recent years, to meet the demand for natural lactones, many microbiological processes have been developed in which natural precursors of lactones, comprising hydroxylated derivatives of natural fatty acids, are degraded to gamma and delta lactones depending on the position of the hydroxyl in relation to the carboxyl in the precursor used.
Gamma decanolide is produced in this way from ricinoleic acid (U.S. Pat. No. 4,560,656 and European Patent EP-B-0 258 993). Other lactones with an even number of carbon atoms are obtained by the microbial biodegradation of the products of photooxidation/reduction of oleic, linoleic and linolenic acids as described in EP 90402217.5.
In all these cases, the lactones obtained contain an even number of carbon atoms. This is due to factors affecting biosynthesis. The hydroxylated natural precursors obtained from unsaturated C.sub.18 or C.sub.16 acids have an even number of carbon atoms and, if the degradation is effected by beta oxidation with the removal of two carbon atoms as S-acetyl CoA, lactones with an even number of carbon atoms are obtained.
The proposed degradation scheme is as follows: ##STR1##
However, gamma and delta lactones with an odd number of carbon atoms exist in nature. Of these, gamma-nonanolide (1) and gamma-2-nonenolide (2) are particularly important; their structural formulae are shown below: ##STR2##
These two products are minor constituents of Dipteryx odorata (tonca bean) (M. Woerner et al. in Unters. Forsch. 1991, 193, 21). Gamma-nonalactone is also a known constituent of the apricot, the strawberry, the blackberry, the peach and many other fruits. In the synthetic racemic form, gamma nonanolide is widely used in the food flavouring industry, for example as a constituent of fruit flavours. However, despite the importance of these lactones with an odd number of carbon atoms, there are no natural processes capable of supplying substantial quantities of product in an optically active form.